A cultured epithelial cell line which retains specific electrolyte transport functions may serve as a model system to study those functions. Culture cells allow the transport processes to be studied in a homogeneous group of cells free of the confounding effect of other cell types. This is an advantaje which should allow better and more specific identification of the sequence of cellular events involved in a transport phenomenon, beginning with receptor binding to activation of intracellular amplifying mechanisms to stimulation of various transport pathways on the plasma membrane. Initial characterization of receptor-mediated ion transport properties in the T84 colonic cell line, an established human epithelial cell line, suggests that this cell line may serve as a useful secretory model system. In this proposal, the role of Cl- and K+ channels and Na+,K+,Cl- cotransport in cAMP and Ca++ mediated secretion will be further investigated in T84 cells. Initial studies suggest that these transport pathways are intimately involved in secretory mechanisms. Three methods will be used to study ion transport. 1) The Ussing chamber, to study transepithelial movement of ions. Synergism between cAMP and Ca++ mediated secretion will be tested, and the involvement of transport pathways and intracellular mediators in secretory processes will be studied. 2) Radionuclide uptake and efflux in whole cells to directly verify the specific transport pathways involved and to study their interactions. 3) Transport studies in purified brush border and basolateral membranes to localize transport pathways and determine its transport characteristics. The proposed studies should allow us to identify various transport pathways on the plasma membrane involved in the secretory events and provide information about the intracellular amplifying mechanisms. Information on the inhibition of transport pathways and the interruption of intracellular amplifying mechanisms should provide a rational approach to the development of antidiarrheal medications.